Driven by the Moore's Law, a feature size of the conventional MOSFET continues to shrink and now enters into a nanometer scale. Consequently, negative effects such as short channel effect become more serious. Besides, effects such as drain induced barrier lowering and band-to-band tunneling cause an off-state leakage current to be continually increased. At the same time, a sub-threshold slope of the conventional MOSFET is not able to be decreased in synchronization with the shrink of the size of the MOSFET due to the limitation by the thermal potential, and thereby the power consumption increases. The concern of the power consumption now becomes the most serious problem limiting the scaling down of the MOSFET.
In order to be applied to the field of ultra-low voltage and ultra-low power consumption, a device having an ultra-steep sub-threshold slope, obtained by adopting a new turning-on mechanism and a fabrication method thereof have gained attentions in the context of small size devices. In recent years, researchers have proposed a possible solution, that is, a tunneling field effect transistor (TFET). Different from the conventional MOSFET, the TFET has source and drain regions doped with opposite types and achieves turning-on by controlling the band-to-band tunneling of the reverse-biased P-I-N junction through the gate, thereby breaking through the limitation of the sub-threshold slope 60 mV/dec of the conventional MOSFET while generating a very small leakage current. The TFET has several superior characteristics such as low leakage current, low sub-threshold slope, low operating voltage and low power consumption. However, due to the limitation of the tunneling probability and the tunneling area for the source junction, the TFET is faced with a problem of small on-state current, which is far less than that of the conventional MOSFET, and this greatly limits the application of the TFET. In addition, the TFET having a steep sub-threshold slope is difficult to be achieved in experiments. This is because it is difficult in experiments to achieve a steep doping concentration gradient at the source junction, so that the electric field at the tunneling junction is not sufficiently large when the TFET turns on, causing the sub-threshold slope of the TFET to be degraded relative to the theoretical value. Therefore, it has become another important issue of the TFET that how to achieve a steep doping concentration gradient at the source junction in order to obtain an ultra-low sub-threshold slope.